Allow passing null for customElementRegistry

Apparently people are passing a ShadowRoot instance as the argument to attachShadow().

Context: https://bugs.webkit.org/show_bug.cgi?id=295174. Tests are added as part of that fix.

Author: annevk

dom.bs

@@ -6598,10 +6598,14 @@ dictionary ShadowRootInit {
   SlotAssignmentMode slotAssignment = "named";
   boolean clonable = false;
   boolean serializable = false;
-  CustomElementRegistry customElementRegistry;
+  CustomElementRegistry? customElementRegistry = null;
 };
 </pre>
 
+<p class=note>{{ShadowRootInit}} somewhat unusually allows both <code>undefined</code> and
+<code>null</code> to be passed to its {{ShadowRootInit/customElementRegistry}} member to allow web
+developers to pass a {{ShadowRoot}} node instead of a dictionary to {{Element/attachShadow()}}.
+
 <p>{{Element}} <a for=/>nodes</a> are simply known as
 <dfn export id=concept-element lt=element>elements</dfn>.
 
@@ -7509,8 +7513,8 @@ are:
 <ol>
  <li><p>Let <var>registry</var> be <a>this</a>'s <a for=Element>custom element registry</a>.
 
- <li><p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] <a for=map>exists</a>, then
- set <var>registry</var> to it.
+ <li><p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] is non-null, then set
+ <var>registry</var> to it.
 
  <li><p>Run <a>attach a shadow root</a> with <a>this</a>,
  <var>init</var>["{{ShadowRootInit/mode}}"], <var>init</var>["{{ShadowRootInit/clonable}}"],